The meat packing industry may be commonly divided into three segments. They are fresh meats, frozen meats and processed meats. This invention relates to fresh red meats, such as beef and pork, which is distinct from fresh white meat, such as poultry. This invention also relates to processed meats.
In the Fresh Meat Industry, the cattle and swine are slaughtered and broken down into primal and subprimal meat cuts. The primal and subprimal meat cuts are large cuts of meat. They are smaller than a side of beef, for example, but larger than the ultimate cut which is sold at retail to the consumer. A primal cut comprises the entire section of a side of beef, such as the rib section or the rump roast section, while a subprimal cut comprises only a portion of such a section. Primal and subprimal cuts are prepared at the slaughter house and are then shipped to a retail meat store, or to an institution such as a hospital, hotel or restaurant, where they are butchered into small cuts of meat suitable for the individual consumer.
The Processed Meat Industry takes various portions of the animal carcasses and processes these portions under varying conditions to produce finished meat products which may be used directly by the consumer. Products may include ham, smoked picnics, smoked butts, corned beef, turkey breast, and various sausage products such as frankfurters, smoked sausage links, bologna, salami, and the like. These products may be packaged in consumer portions or they may be packaged in bulk for shipment to a retail meat store, restaurant or hotel. Bulk shipments may include such items as ham chunks, cooked turkey breasts, bologna chubs, long bologna for delicatessen sale, rings of bologna, corned beef brisket, smoked picnics, smoked butts and linked products such as smoked sausage.
When fresh red meat cuts, such as roast or rib sections, and bulk processed meats are prepared for shipment or storage, they are usually packaged in such a way that air (i.e., oxygen) is prevented from contacting the meat and moisture is prevented from leaving the meat. This is done in order to minimize spoilage and discoloration during shipping and handling. One desirable way to package fresh red meats and processed meats so as to protect them from contact with air and from moisture loss is to shrink package them with a packaging material that has good oxygen and moisture vapor barrier properties. One such shrink packaging material that has good oxygen and moisture vapor barrier properties is polyvinylidene chloride film. Vinylidene chloride-vinyl chloride copolymers are commonly referred to as PVDC.
While vinylidene chloride-vinyl chloride copolymer film has excellent barrier properties, in actual practice, when PVDC is used as a monolayer film, it must be plasticized in order for the film to have adequate abrasion resistance and flexibility at storage temperatures of, for example, 30.degree. to 50.degree. F. Unfortunately, the addition of plasticizer sufficient to provide the requisite low temperature properties to the PVDC monolayer film has a significant adverse effect on the barrier properties of the film. While increasing the thickness of the film from the conventional thickness of 1.5 to 2.0 mils, to 5 mils or more, for instance, would improve the barrier properties of the film, it would be economically undesirable to use a monolayer film of PVDC having a thickness of 5 or more mils. Also, if such thick films were employed, bags made from the film would be difficult to gather and clip closed at the open end.
One approach to the provision of a film having barrier properties which are better than those of the 1.5 to 2.0 mil monolayer PVDC film previously used for shrink packaging meat, is to employ a multilayer film, one layer of which is vinylidene chloride-vinyl chloride copolymer having a minimum amount of plasticizer. The other layer or layers of such multilayer films are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in the vinylidene chloride-vinyl chloride layer containing little or no plasticizer.
In providing such a film, however, it must be recognized that good oxygen and moisture vapor barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging processed meats and primal and subprimal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient for the film to heat-shrink within a specified range of percentages, e.g., from about 15 to 60 percent at about 90.degree. C., in both the machine and the transverse directions. (Conventionally, the term "MD" refers to machine direction and the term "TD" refers to transverse direction.) The film must be heat sealable in order to be able to fabricate bags from the film and in order to heat seal the open ends of the fabricated bags after insertion of the meat product. The heat sealed seams of the bags must not pull apart during the heat shrinking operation, and the film must resist puncturing by sharp bone edges during the heat shrinking operation.
Also, there must be adequate adhesion between the several layers of the film so that delamination does not occur, either during the heat shrinking operation or during exposure of the film to the relatively high temperatures that may be reached during shipping and storage of the film in the summertime. Delamination is the phenomenon where layers of the multilayer film are readily separable or easily pulled apart from face to face integrity with no tearing of the individual layers of film.
In order to overcome problems of delamination, it is known in the prior art to use adhesive layers between layers which do not otherwise adhere to each other with the required bonding strength under normal conditions of commercial use. For example, polyethylene and polypropylene are known to have poor lamination characteristics when in face to face relationship with the oxygen barrier layers conventionally used in bags of multilayer films used for packaging primal or subprimal meat cuts of fresh red meat, and in bags for packaging processed meats. Adhesive layers are often used in compensation for such poor lamination characteristics.
By way of illustrating multilayer film containing adhesive layers, Shirmer U.S. Pat. No. 4,448,792 teaches a cook-in shrink bag fabricated from a multilayer film having a first heat sealing and food contacting layer composed of propylene homopolymer or copolymer; a second heat shrinkable layer composed of a blend of propylene homopolymer or copolymer and butylene homopolymer or copolymer; a third adhesive layer composed of irradiatively cross-linkable ethylene copolymer; a fourth oxygen barrier layer comprising vinylidene chloride copolymer; a fifth adhesive layer of irradiatively cross-linkable ethylene copolymer; and a sixth optical clarity layer comprising propylene homopolymer or copolymer; wherein the entire six layer film has been irradiated either before or after the multilayer film has been biaxially stretched. Among the several key benefits enumerated by Schirmer, the completely irradiated six layer film provides bags having structural integrity in that the bags resist delamination and their heat seals have high temperature resistance, while at the same time the bags are acceptable for food contact in terms of minimum levels of extractables. Cook-in bags made of this multilayer film will maintain seal integrity and will resist delamination when submerged in water at 80.degree. C. for 12 hours.
While the use of adhesive layers, as taught in such prior art, is effective in avoiding problems of delamination, it is an undesirable solution. First of all, additional equipment is required, and the process becomes more complex with the need for new extruders and extrusion dies. Additionally, the addition of adhesive layers will generally make the film thicker. The production of a thicker film may be avoided, however, by reducing the thickness of the other film layers or by controlling the thickness of the adhesive layers so that they are very thin. Since the oxygen barrier layer thickness may not be reduced without the loss of barrier effectiveness, it becomes necessary to only reduce the thickness of the outer layers, and this can cause a reduction in heat sealability and/or a reduction in puncture resistance. Thus, it is preferred not to reduce the thickness of the outer film layers, but to control the adhesive layers to a minimum thickness which is effective in bonding the layers sufficiently to avoid delamination problems. This in turn requires sophisticated equipment, which is expensive, since the equipment must control the adhesive layer thickness generally to a range of from about 0.10 to about 0.15 mil. Moreover, the adhesives themselves are generally very expensive, and the cost of adhesive plus the cost of the new equipment generally causes an increase in the price of the bags produced from the multilayer film.
Thus, it is preferred to find a means for strongly bonding the other layers directly to the oxygen barrier layer without the use of prior art adhesive layers.
It should be noted that the aforementioned Shirmer U.S. Patent No. 4,448,792 teaches that both polypropylene and blends of propylene with another polymer require an adhesive layer in order to bond these layers acceptably to the oxygen barrier layer of vinylidene chloride copolymer. However, the prior art also teaches such a need for adhesive layers in regard to polyethylene.
Illustrative of a polyethylene, unblended with any other polymer, which requires an adhesive layer in order to acceptably bond to the oxygen barrier layer is U.S. Pat. No. 4,640,856 to Ferguson et.al. This patent discloses bags for the packaging of fresh red meat (primal and subprimal meat cuts), cheeses, poultry and other food and non-food products, wherein the bag is fabricated from a multilayer thermoplastic shrink film having a substrate layer of a very low density polyethylene which has been extrusion coated with at least a gas barrier layer comprising a copolymer of vinylidene chloride or a hydrolized ethylene vinyl acetate copolymer. This two layer film is then extrusion coated with another layer of a thermoplastic polymer to form at least a three layer film having a core layer of the gas barrier layer. Ferguson et.al. teach that an ethylene vinyl acetate layer should be interposed between the surface layer of very low density polyethylene and the gas barrier core layer of vinylidene chloride copolymer to promote adhesion between the layers and to lessen any tendency of the film to delaminate, since very low density polyethylene does not adhere to vinylidene chloride copolymers as well as the ethylene vinyl acetate does. Additionally, this patent teaches that for maximum delamination protection adhesive layers should be used to bond outer layers of very low density polyethylene to core layers of barrier film, both when the oxygen barrier layer is a copolymer of vinylidene chloride and when it is a hydrolyzed ethylene vinyl acetate.
Illustrative of a polyethylene blended with another polymer in one layer of a multilayer film, wherein an adhesive layer is required in order to acceptably bond the blend layer to the oxygen barrier layer, is U.S. Pat. No. 4,456,646 to Nishimoto et.al. This patent teaches that a multilayer film for the packaging of meats and cheeses may be fabricated of a core layer of a vinylidene chloride copolymer with outer layers of a blend of ethylene vinyl acetate and a linear low density polyethylene having a density of from 0.900 to 0.950 grams per cubic centimeter. Nishimoto et.al. teach that the linear low density polyethylene, which is a copolymer of ethylene with an alpha olefin having less than 18 carbon atoms, will not adhere to the core layer of vinylidene chloride copolymer so that the outer layers are apt to delaminate from the core layers, particularly in an elevated temperature environment. Accordingly, Nishimoto et.al. teach that it is necessary to provide an adhesive layer between the core layer of barrier film and each outer layer containing the blend of ethylene vinyl acetate and linear low density polyethylene.
We have found that not only are the multilayer film structures of Ferguson et.al. U.S. Pat. No. 4,640,856 undesirable from the standpoint of the adhesive layer requirements, but in fact, contrary to the teachings of Ferguson et.al., we have discovered that a first outer layer of very low density polyethylene will bond directly to a core layer of oxygen barrier film comprising a vinylidene chloride copolymer without the use of any adhesive layer interposed therebetween, if the multilayer film is a coextruded film. This matter will be discussed more fully hereinafter.
We have also found that not only are the polyethylene blends of Nishimoto et.al. U.S. Pat. No. 4,456,646 undesirable from the standpoint of the adhesive layer requirement, but they are also undesirable because they cause the optical properties of the film to be unacceptably degraded. Such polyethylene blends cause the haze value for the multilayer film to increase to an unacceptable level, and they cause the gloss value for the multilayer film to decrease to an unacceptable level.
The haze value is important because it is an indication of the ability of the film to transmit light. A low haze value indicates a very clear film which enables one to clearly see the contents of the package. We find that the haze value must not exceed 6.5% when packaging fresh red meat cuts and processed meats. This value is particularly important in regard to the packaging of processed meats, since it is the individual consumer who is viewing the package and deciding whether or not to make a purchase.
The gloss value is important because it is a measure of the shiny appearance of the film. A high gloss value indicates that the packaged meat product will have a very shiny highly attractive appearance. We find that the gloss value should not be below 70% when packaging fresh red meat cuts and processed meats. This value is particularly important in regard to the packaging of processed meats, since it is the individual consumer who is viewing the package at the point of purchase.
Another important consideration in evaluating multilayer films is the type and degree of film curl which the film exhibits. Film curl is an indication of the ease or the difficulty which is experienced in opening a bag which has been fabricated from the multilayer film. In order for a multilayer film to be commercially acceptable, the film must be capable of producing bags which are easily opened by the bagging operator who places the meat product into the bag on the production line of the meat packing plant. Bags which are difficult to open cause delays in the bagging operation and result in low production efficiency. The significance of film curl will be discussed more fully hereinafter.
In summary then, it is an object of the present invention to provide a multilayer film containing a core layer of an oxygen barrier and outer layers of polyethylene, wherein the outer layers are bonded directly to the core layer with no adhesive layers interposed therebetween.
It is another object of the present invention to provide such multilayer films, wherein the haze and gloss properties of the multilayer film are acceptable under conditions of commercial use.
It is a further object of the present invention to provide such multilayer films, wherein the films are capable of fabrication into bags which are easily openable under conditions of commercial use, as indicated by the curl properties of such multilayer films.
It is a still further object of the present invention to provide such multilayer films, wherein no adhesive layers are employed to bond the outer layers to the core layer, and wherein the outer layers provide improved strength characteristics and improved puncture resistance to the film.